Help understanding how this routing setup works

Hi All,

While having a basic understanding of how routing stuff works I don't seem to be able get my head around this one.

I have a branch router with 2 FE interfaces connected like this:

FE0/0 ---> WAN access device (192.168.1.105) FE0/1 ---> LAN (192.168.1/24)

Then relevant configuration is like this:

interface FastEthernet0/0 ip address 192.168.1.1 255.255.255.0 duplex auto speed auto

interface FastEthernet0/1 no ip address duplex auto speed auto

interface FastEthernet0/1.1 encapsulation dot1Q 1 native ip unnumbered FastEthernet0/0

ip route 0.0.0.0 0.0.0.0 192.168.1.105 ip route 192.168.1.105 255.255.255.255 FastEthernet0/1.1

When the las t route is not present the WAN cannot be reached. Why is it needed? I can see some VLAN config on FE0/1.1. Is it really needed?

In general how does this work? show ip route shows:

ip route 0.0.0.0 0.0.0.0 192.168.1.105 ip route 192.168.1.105 255.255.255.255 FastEthernet0/1.1

Here is how I (don't) understand it. Say a packet arrives at FE0/1.1 for some WAN destination. Then it is forwarded to 192.168.1.105 as per default route. But there is a host route which puts this packet through FE0/1.1. So how does it reach the WAN if it is phisically connected to FE0/0 and not to FE0/1?? More, why this packet does not reach the WAN if not forced through the (wrong) phisical interface?

And last. Why should such a setup be preferable over a simpler one like:

interface FastEthernet0/0 ip address 192.168.1.1 255.255.255.0 duplex auto speed auto

interface FastEthernet0/1 ip address 192.168.1.2 255.255.255.0 duplex auto speed auto

ip route 0.0.0.0 0.0.0.0 192.168.1.105 ip route 192.168.1.105 255.255.255.255 FastEthernet0/0 ip route 192.168.1.0 255.255.255.0 FastEthernet0/1

Thanks in advance!

Reply to
David Requena
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The reason that you cannot figure it out is that no sensible person would ever use such a configuration.

It must have been done for a workaround of some sort.

you say:-

"And last. Why should such a setup be preferable over a simpler one like:

interface FastEthernet0/0 ip address 192.168.1.1 255.255.255.0 duplex auto speed auto

interface FastEthernet0/1 ip address 192.168.1.2 255.255.255.0 duplex auto speed auto

Well this is actually the FIRST question.

No Cisco router will accept this invalid configuration.

Different interfaces MUST be on different networks so the router will not accept such a configuration.

IP unnumbered is not intended to be used on broadcast media such as Ethnernet. It is valid on point to point links such as serial interfaces.

If it is working then I salute the original implementor. I cannot figure it out. It looks as if the router has been configured to wedge in where a bridge is actually what is needed. You can of course configure the router as a bridge if required.

Perhaps the rest of the config might have some clues. Please post the whole config. Also perhaps describe the rest of the network. Perhaps the router can simply be removed?

Reply to
bod43

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